1. Field of the Invention
The present invention relates to a construction of a combustion chamber for an in-cylinder direct fuel injection type spark ignition engine and more particularly to a construction of a cylinder head and a piston so as to readily produce a tumble flow and to a disposition of a spark plug and a fuel injector so as to optimize the combustion of fuel sprayed from a fuel injector.
2. Prior Arts
Generally, in this type of engine the combustion strategy can be selectively switched to either of two combustion strategies, a stratified charge combustion or a homogeneous combustion. The stratified charge combustion is obtained by accomplishing charge stratification at the later stage of the compression stroke and forming ignitable mixture gases around the spark plug. The homogeneous charge combustion is achieved by mixing fuel injected during the intake stroke with intake air.
As an example of the technology of the combustion chamber suitable for both stratified charge combustion and homogeneous charge combustion, an inventor of the present invention has proposed in Japanese Patent Application Laid-open No. Toku-Kai-Hei 6-42352 a fuel injector disposed in a vertical position at the top center of the combustion chamber and a cavity formed at the top surface of the piston in the counter direction to the injection direction of the fuel injector. Further, in this invention an electrode of the spark plug is disposed in the vicinity of the nozzle of the fuel injector.
According to this combustion chamber structure, with the stratified charge combustion, an end portion of fuel finishing injection immediately before the ignition timing is ignited by the spark plug or air-fuel mixtures impacted on and reflected by the cavity of the piston are ignited just at that ignition timing, whereby combustion stability is secured. On the other hand, with the homogeneous charge combustion, since fuel is started to be injected at a rather early stage of the intake stroke, homogeneous air-fuel mixtures can be obtained. Furthermore, according to this combustion chamber, since the fuel injector is provided in a vertical position at the top center of the combustion chamber, sprayed fuel is prevented from sticking to the cylinder wall surface, being able to thus avoid an adverse effect on combustion due to a so-called fuel quenching.
In order to obtain a stable combustion by forming ignitable air-fuel mixtures at the ignition point of the spark plug, a best finishing timing of fuel injection (BITI) where ignition is best has a characteristic shown by a broken line in FIG. 13. As understood by the characteristic line, the best fuel injection timing has a very weak correlation with respect to the fuel injection amount (or engine load) and consequently it is understood that the finishing timing of fuel injection is allowed to be kept constant with respect to the ignition timing.
However, as shown in FIG. 14, when the fuel injection timing gets advanced (rendered early) with respect to the ignition timing, since injected fuel is more premixed with air, HC and NOx emissions increase up to a given advance angle and then thereafter gradually decrease. On the contrary, when the fuel injection timing gets retarded (rendered late) with respect to the ignition timing, since injected fuel is burned in droplets, smoke emissions increase due to the lack of vaporization of fuel. Further, there is a tendency that the timing of smoke generation comes early as the fuel injection amount increases. Consequently, according to a best injection timing control (BITE), it is necessary from the aspect of emissions (smoke, CO, HC and NOx) that the fuel injection timing be advanced with an increase of the fuel injection amount (engine load).
As a result of this, in the stratified charge combustion, it is clearly understood that the BITI control focused on ignitability has a different control area from the BITE control focused on emissions countermeasures.
However, in the case of a combustion chamber wherein a flat cavity la is formed on the top surface of the piston as shown in FIG. 15, if the fuel injection (finishing) timing is established early, fuel sprayed from the fuel injector 2 is diffused around after being impacted on the cavity. As a result, fuel spray does not reach the neighborhood of the electrode 3a of the spark plug 3 and ignitable air-fuel mixtures can not be formed around the electrode 3a, leading to misfires or incomplete combustion. Therefore, in the case of an engine having a piston configuration like this, it is necessary to bring the injection finishing timing close to the ignition timing and to ignite an end portion of sprayed fuel. That is to say, it is understood that there is a limit in advancing the fuel injection (finishing) timing according to the fuel injection amount.
To solve this problem, there is an idea that the flat cavity 1a at the top surface of the piston is modified to a curved shape so that fuel injected from the fuel injector curls up along the curved surface of the cavity. This idea enables an ignitable air-fuel mixture to form around the electrode of the spark plug and to advance the fuel injection timing to some degree. However, by only this method relying upon the fuel spray configuration, it is still inadequate to secure ignitable air-fuel mixtures around the spark plug, when the fuel injection timing is further advanced.
Generally, in the case of stratified charge combustion, air-fuel mixtures around the electrode of the spark plug become overrich or cause a lack of vaporization as a mean air-fuel ratio comes near the stoichiometric air-fuel ratio, i.e., the injection amount is increased and this results in the generation of smoke, CO and HC emissions. This means that there is a given rich limit in the mean air-fuel ratio with stratified charge combustion. On the other hand, in the case of homogeneous charge combustion, since the overall mixture of gases is homogenized, there is also a given lean limit under which igniting is impossible.
It is known that the stratified charge combustion is suitable for low and medium load operations and homogeneous charge combustion is suitable for high load operations. Engine loads vary continuously during operation. Further, in the in-cylinder direct fuel injection engine air-fuel ratios are established variably according to changing engine loads. Therefore, in a case where the rich limit of the stratified charge combustion is located at a leaner side than the lean limit of homogeneous charge combustion, each time the operating area of engine varies, the air-fuel ratio is changed discontinuously, that is, when changing from stratified charge combustion to homogeneous charge combustion, a swift change occurs in the rich direction of the air-fuel ratio and when changing from homogeneous charge combustion to stratified charge combustion, a rapid change occurs to the lean side.
Thus, the situation that the air-fuel ratio is changed discontinuously each time the combustion strategy varies incurs deteriorated emissions and unacceptable driveability. This situation will be described with reference to FIG. 13.
In order to retain the continuity of the air-fuel ratio (assuming that the intake air amount is constant), if the injection amount with stratified charge combustion is established at the same level P.sub.1 as the lean limit P.sub.3 of homogeneous charge combustion, there occur inconveniences such as a generation of smoke emissions and an increase of CO emissions at the point P.sub.1. On the other hand, if the injection amount is established at the rich limit P.sub.2 so as to avoid these inconveniences when switching from stratified charge combustion to homogeneous charge combustion, the injection amount is increased from P.sub.2 to P.sub.3 abruptly and this brings about a discontinuity of engine output against engine load.